1. Field of the Invention
The present invention relates to a magneto-optic overwrite disk recording system for recording data by overwrite, i.e., by writing new data over old data.
2. Description of the Related Art
Magneto-optic disks are receiving great attention as memory device playing an important role in the field of rapidly progressing multimedia equipment and there are strong demands for development of magneto-optic overwrite disks capable of transferring data at higher speeds. An ordinary magneto-optic disk has only one layer of magnetic film. Hence, when data is to be written in the disk, it is required to initialize the disk by aligning the magnetization of the magnetic film in one direction before writing data. To read one track of data with the magneto-optic disk of the described type, the disk is only required to be rotated one rotation. However, when writing data, since the initialization is necessary, the disk must be rotated at least two rotations, and because of this, the data transfer speed has been slow.
As one of the measures to achieve high-speed data transfer, it is urgently demanded to develop a magneto-optic disk recording system using a magneto-optic overwrite medium capable of writing data through rotation of the disk only one rotation.
Referring to FIG. 1, a case of recording data in a magneto-optic overwrite disk will be described. A magneto-optic overwrite disk 2 is constructed of a memory magnetic film 6 formed on a transparent substrate 4 and a record magnetic film 8 formed on the memory magnetic film 6. The memory magnetic film 6 and the record magnetic film 8 are vertically magnetized films having different temperature characteristics of coercive force and Curie points and they are laminated to each other to achieve exchange-coupling therebetween. A laser beam 18 generated by a collimated laser beam generator 12 is reflected by a mirror 14 of an optical head 10 and focused through an objective lens 16 on the memory magnetic film 6 of the magneto-optic overwrite disk 2. On the side of the disk 2 opposite to the side irradiated by the laser beam, there is disposed a biasing magnet 22 applying a bias magnetic field Hb perpendicularly to the magnetic films 6 and 8, and on the upstream side of the biasing magnet 22, there is disposed an initializing magnet 20 applying an initialization magnetic field Hi to the magnetic films 6 and 8 in the opposite direction to the magnetic field of the biasing magnet 22.
The overwrite of data is achieved by utilizing the difference in the temperature characteristics of the coercive force between the exchange-coupled memory magnetic film 6 and the record magnetic film 8. FIG. 2 is a diagram showing the temperature characteristics of the coercive force of the memory magnetic film 6 and the record magnetic film 8. The broken line 24 shows the coercive force of the memory magnetic film 6 and the solid line 26 shows the coercive force of the record magnetic film 8. As apparent from the diagram, both the memory magnetic film 6 and the record magnetic film 8 have their coercive forces decreasing with increase in the temperature. At the room temperature Ta, the coercive force of the memory magnetic film 6 is greater than the strength of the initialization magnetic field Hi and the coercive force of the record magnetic film 8 is smaller than the strength of the initialization magnetic field Hi.
The coercive force of the memory magnetic film 6 is smaller than the strength of the bias magnetic field Hb at the temperature higher than the first elevated temperature T1, while the coercive force of the record magnetic film 8 is smaller than the bias magnetic field Hb at the temperature higher than the second elevated temperature T2. The temperature range below the first elevated temperature T1 is defined as a first range, the temperature range above the first elevated temperature T1 and below the second elevated temperature T2 is defined as a second range, and the range above the second elevated temperature T2 is defined as a third range. The coercive force of the memory magnetic film 6 and the coercive force of the record magnetic film 8 intersect each other in the first range. The memory magnetic film 6 has its Curie point C1 in the second range, while the record magnetic film 8 has its Curie point C2 in the third range.
In the case where an ordinary magneto-optic disk with one layer of magnetic film is used, a laser beam with high power has been used for irradiating the disk surface in the recording of data and a laser beam with low power has been used for irradiating the disk surface in the reproduction of data. However, in the case where a conventional magneto-optic overwrite disk is used, two kinds of laser beams, i.e., that with high power and that with low power, have been used for irradiating the disk surface in the recording of data and a laser beam with lower power than the low power used in the recording of data has been used for irradiating the disk surface in the reproduction of data. The beam power used in the reproduction of data will hereinafter be called "read power".
Namely, in a conventional magneto-optic overwrite disk unit, laser beams with two levels of power, i.e., high power P.sub.H and low power P.sub.L, have been used in the recording of data and a laser beam with read power P.sub.R has been used in the reproduction of data. When the laser beam with the read power P.sub.R is used, the temperature of the irradiated magnetic films 6 and 8 can be kept within the first range, i.e., below the point at which the coercive force of the memory magnetic film 6 indicated by the broken line 24 in FIG. 2 coincides with the strength of the bias magnetic field Hb.
When the laser beam with the low power P.sub.L is used for irradiation, the temperature of the magnetic films 6 and 8 can be elevated to the point, at which the coercive force of the memory magnetic film 6 is below the strength of the bias magnetic field Hb but the coercive force of the record magnetic film 8 indicated by the solid line 26 is not below the bias magnetic field Hb, in the second range (low temperature course). When the laser beam with the high power P.sub.H is used, the temperature of the magnetic films 6 and 8 can be elevated to the temperature in the third range (hightemperature course) at which the coercive force of the record magnetic film 8 is below the strength of the bias magnetic field Hb.
Operations at the time of the overwrite of data will be described below. When a disk 2 is rotated so that the portion to be recorded of the track assumes the position confronted by the initializing magnet 20, only the record magnetic film 8 is initialized by the initialization magnetic field Hi. This is because, as shown in FIG. 2, the strength of the initialization magnetic field Hi at the temperature around the room temperature Ta is set to be weaker than the coercive force of the memory magnetic film 6 and stronger than the coercive force of the record magnetic film 8. Hence, around the room temperature Ta, only the record magnetic film 8 is magnetized in the direction aligned with the initialization magnetic field Hi.
When the disk 2 is further rotated and the initialized portion is brought to the position corresponding to the biasing magnet 22, the recording of data is performed in this position. When the data to be recorded is binary data as shown in FIG. 3(A), the laser diode within the collimated laser beam generator 12 is driven so that the laser beam assumes the pattern of low power P.sub.L and high power P.sub.H as shown in FIG. 3(B) according to the binary data.
When the laser beam is with the low power P.sub.L, the temperature of the magnetic films 6 and 8 becomes that in the second range shown in FIG. 2 and the coercive force of the memory magnetic film 6 becomes weaker than the bias magnetic field Hb. At this time, the coercive force of the record magnetic film 8 is still stronger than the bias magnetic field Hb, and therefore, when the memory magnetic film 6 comes to be magnetized in the following course in which the disk 2 is further rotated and the irradiated portion is cooled down, the direction of magnetization of the memory magnetic film 6 is aligned with the direction of magnetization of the record magnetic film 8 by action of the exchange-coupling (exchange-interaction). That is, when the magnetic films 6 and 8 are heated up to the temperature in the second range, the direction of magnetization of the memory magnetic film 6 is aligned with the direction of the initialization magnetic field Hi.
On the other hand, when the laser beam is with the high power P.sub.H, the magnetic films 6 and 8 are heated up to the temperature in the third range, in which the coercive forces of the memory magnetic film 6 and the record magnetic film 8 both become weaker than the bias magnetic field Hb. Therefore, the direction of magnetization of the record magnetic film 8 is aligned with the direction of the bias magnetic field Hb, which is opposite to the direction of magnetization in the initialization. In the following temperature falling course, the direction of magnetization of the memory magnetic film 6 is aligned with the direction of magnetization of the record magnetic film 8 by the exchange-interaction. More specifically, when the magnetic films 6 and 8 are heated up to the temperature in the third range, the directions of magnetization of both the memory magnetic film 6 and the record magnetic film 8 are aligned with the direction of the bias magnetic field Hb which is opposite to the direction of the initialization magnetic field Hi.
Accordingly, when the magnetic films 6 and 8 are heated up to a temperature in the third range, record marks (magnetic domains) 28 are formed in the memory magnetic film 6 as shown in FIG. 3(C) and, when the magnetic films 6 and 8 are heated up to a temperature in the second range, formerly recorded marks in the memory magnetic film 6 are erased as indicated by reference numeral 30 in FIG. 3(C). In this way, the overwrite of data can be achieved. In the reproduction of data, a laser beam with the read power P.sub.R is thrown on the disk 2 and a reproduced signal corresponding to the direction of magnetization as shown in FIG. 3(D) can be obtained by utilizing the magnetic Kerr effect.
However, when the mark 28 is recorded with the laser beam with the high power P.sub.H and low power P.sub.L, if the front edge 34 of the record mark 28 corresponding to the leading edge 32 of the high level and the rear edge 38 of the record mark 28 corresponding to the trailing edge 36 of the high level shown in FIG. 3(B) and FIG. 3(C) are checked, the rear edge is shifted as indicated by the broken line 40 as a result of more greatly accumulated heat toward the rear of the record mark 28 because the disk 2 is heated by the laser beam while it is rotated. The longer the record mark 28 is, the greater becomes the accumulated heat at the rear edge side, and hence the larger becomes the shift of the rear edge. As a result, a proper mark 28 becomes unable to be recorded.
There is proposed a method to reduce the shift of the rear edge. This method is such as causes the laser beam in the formation of a mark to be emitted as pulsed beam as shown in FIG. 4(B). If the laser beam is emitted as a pulsed beam, the heat energy supplied to the magnetic films 6 and 8 can be suppressed as compared with the case where the laser beam is emitted continuously.
More specifically, when a laser beam with the waveform as shown in FIG. 3(B) is thrown on the disk 2, the heated temperature of the magnetic films 6 and 8 becomes as indicated by the broken line 42 in FIG. 4(C), but when a laser beam with the waveform as shown in FIG. 4(B) is thrown, the heated temperature of the magnetic films 6 and 8 becomes as indicated by the solid line 44 in FIG. 4(C). As a result, the shift of the rear edge of the record mark 46 indicated by the broken line 48 in FIG. 4(D) can be canceled.
A conventional magneto-optic overwrite disk recording system for recording data with a pulsed laser beam will be described below with reference to FIG. 5. The recording system is structured of a data conversion circuit 50, an AND circuit 52, a mono multivibrator 54, switch circuits 56 and 58, a constant-current circuit for high power 60, a constant-current circuit for low power 62, a constant-current circuit for read power 64, and a laser diode 66, which are connected as shown in FIG. 5.
The data conversion circuit 50 converts record data D1 with the waveform as shown in FIG. 6(B) to data D2 with the waveform as shown in FIG. 6(D). The record data D1 is that obtained by converting the binary data shown in FIG. 6(A) to a waveform having high level and low level corresponding to "1" and "0" in the binary data and the record data D1 corresponds to the data shown in FIG. 4(A). The AND circuit 52 produces the logical product of the data D2 and the clock signal CK and outputs the logical product as data D3, which has a waveform as shown in FIG. 6(E).
The mono multivibrator 54 is that outputting a pulse of a predetermined duty ratio (a pulse with a predetermined pulse width) upon receipt of a piece of high level data. When each pulse of the data D3 is supplied thereto, it, slightly increasing the duty ratio of the data D3, outputs a pulse as shown in data D4 of FIG. 6(F). The switch circuit 56 is turned on when each high level of the data D4 is supplied thereto, while the switch circuit 58 is turned on when each high level of erase data D5 is supplied thereto.
The laser diode 66 selectively outputs laser beams with the read power P.sub.R, the low power P.sub.L, and the high power P.sub.H. When the switch circuits 56 and 58 are both turned off, a constant current controlled by the constant-current circuit for read power 64 is supplied to the laser diode 66 and, hence, the laser diode 66 outputs a laser beam with the read power P.sub.R. Thereby, the marks recorded in the memory magnetic film 6 are read. When the erase data D5 at high level is supplied to the switch circuit 58 and thereby the switch circuit 58 is turned on, a constant current controlled by the constant-current circuit for low power 62 and the constant-current circuit for read power 64 is supplied to the laser diode 66 and, hence, the laser diode 66 outputs a laser beam with the low power P.sub.L. Thereby, the recorded mark in the memory magnetic film 6 is erased.
On the other hand, if the high level of the data D4 is supplied to the switch circuit 56 and thereby the switch circuit 56 is turned on, while the erase data D5 is at high level, a constant current controlled by the constant-current circuit for high power 60, the constant-current circuit for low power 62, and the constant-current circuit for read power 64 is supplied to the laser diode 66, so that the laser diode 66 outputs a laser beam with the high power P.sub.H. Thus, a mark is recorded in the memory magnetic film 6.
With the above described conventional recording system, the shift of the rear edge of the record mark can be canceled. However, there is a problem in this recording system that the front edge of the mark shifts forward making it impossible to record a proper mark. This trouble is produced, when the distance between a record mark and the following record mark is small, by the heat accumulated in the former record mark affecting the front edge portion of the latter record mark thereby causing the front edge of the latter record mark to shift forward.
Further, in the conventional recording system, the recording of data is performed by heating the magnetic films 6 and 8 with laser beams having the high power P.sub.H and the low power P.sub.L up to a temperature in the second range and a temperature in the third range shown in FIG. 2, respectively, and therefore two systems of circuits, i.e., the constant-current circuit for high power 60 and the constant-current circuit for low power 62, are required to drive the laser diode 66 in the data recording. Since the constant-current circuit occupies a large portion of the overall circuit scale of the recording system, there arises a problem that the circuit configuration of the entire system becomes complex and large in scale.